A high-intensity discharge lamp can obtain a high-brightness luminous flux output by means of a compact shape, and is close to point-source light, in which a light distribution control is easy. Accordingly, the high-intensity discharge lamp has come recently to be used as an alternative of an incandescent lamp or a halogen lamp. In general, it is considered that a pulse of a voltage as high as several kilovolts is required for a voltage necessary to start the high-intensity discharge lamp. As shown in FIG. 6, this high-intensity discharge lamp has a circuit configuration that is typical as a conventional example.
Reference numeral 1 denotes a direct-current power supply, reference numeral 2 denotes a DC/DC converter, and reference numeral 3 denotes a DC/AC inverter. An inverter L2 and a capacitor C2 compose a resonant circuit. Moreover, reference numeral 2C denotes a control circuit for the DC/DC converter 2, and reference numeral 3C denotes a control circuit for the DC/AC inverter 3.
The DC/DC converter 2 is composed of a switching element Q1, a diode D1 and an inductor L1. The DC/DC converter 2 charges, to a smoothing capacitor C1, a voltage dropped by performing voltage conversion for a current from the direct-current power supply 1. A both-end voltage of the capacitor C1 is substantially equal to a lamp voltage, and is also easy to detect. Therefore, the control circuit 2C detects the voltage of the capacitor C1 in place of detecting the lamp voltage, and outputs a drive signal of the switching element Q1 in response to a value of the detected voltage. Note that the DC/DC converter 2 in this conventional example is a so-called step-down chopper circuit, and operations thereof are very common, and accordingly, a description of the operations is omitted.
Next, the DC/AC inverter 3 is a full-bridge circuit composed of switching elements Q2 to Q5. A discharge lamp La is connected to an alternating-current output side of the DC/AC inverter 3 through a starting circuit composed of the resonant circuit formed of the resonant inductor L2 and the resonant capacitor C2.
The DC/AC inverter 3 is controlled by the control circuit 3C. The control circuit 3C is composed, for example, of a controlling microcomputer. Operations of the control circuit 3C are described with reference to a flowchart of FIG. 7, and operations of the switching elements Q2 to Q5 and a change of the lamp voltage are described with reference to FIG. 8.
First, during a period from the time of starting the discharge lamp La to the point of time when a predetermined time T elapses, as shown in FIG. 7, the control circuit 3C controls the operations of the DC/AC inverter 3 to pass through a high-frequency operation (Step S101 (HF1)), a square-wave operation (Step S102 (FB1)), a high-frequency operation (Step S104 (HF2)) and a square-wave operation (Step S105 (FB2)), and then to return to the high-frequency operation (HF1).
In the high-frequency operation (HF1), the control circuit 3C is allowed to perform an alternate switching operation at a high frequency between a state where the switching elements Q2 and Q5 are turned on and the switching elements Q3 and Q4 are turned off and a state where the switching elements Q2 and Q5 are turned off and the switching elements Q3 and Q4 are turned on. In such a way, the DC/AC inverter 3 generates a high-frequency and high-voltage pulse by the inductor L2 and resonant capacitor C2 of the resonant circuit.
The square-wave operation (FB1) is an operation in which the control circuit 3C is allowed to turn on the switching elements Q2 and Q5 and to turn off the switching elements Q3 and Q4. This square-wave operation (FB1) is continued for a period of several milliseconds in Step S103.
In the high-frequency operation (HF2) performed after the square-wave operation (FB1) is continued for several milliseconds, the control circuit 3C is allowed to perform an alternate switching operation at the high frequency between a state where the switching elements Q2 and Q5 are turned on and the switching elements Q3 and Q4 are turned off and a state where the switching elements Q2 and Q5 are turned off and the switching elements Q3 and Q4 are turned on. In such a way, the DC/AC inverter 3 generates a high-frequency and high-voltage pulse by the resonant inductor L2 and resonant capacitor C2 of the resonant circuit.
In the square-wave operation (FB2), the control circuit 3C is allowed to turn on the switching elements Q2 and Q5 and to turn off the switching elements Q3 and Q4. This square-wave operation (FB2) is continued for a period of several milliseconds in Step S106.
The control circuit 3C that operates in accordance with such a flowchart drives the switching elements Q2 to Q5 as shown in FIG. 8. In such a way, the control circuit 3C allows the discharge lamp La to cause a dielectric breakdown by high-frequency pulse voltages VP made by the high-frequency operations HF1 and HF2, and during subsequent periods while the square-wave operations FB1 and FB2 are being performed, allows discharge of the discharge lamp La to shift from glow discharge to arc discharge, and thereby starts to light the discharge lamp La.
At the high-frequency operations HF1 and HF2, in the case where the discharge lamp La is not lighted, the high-frequency pulse voltages reach a high voltage value VP, and in the case where the discharge lamp La is lighted, the high-frequency pulse voltages fall to a low voltage value VP′. The reason why the high-voltage pulse voltages fall to the low value when the discharge lamp La is lighted is that a lamp current is restricted by the resonant inductor L2. Moreover, during the periods of the square-wave operations FB1 and FB2, in the case where the discharge lamp La is not lighted, the lamp voltages reach a high voltage value VH, and in the case where the discharge lamp La is lighted, the lamp voltages fall to a low voltage value VL. The above-described operations are repeated during the predetermined period T, and after the point of time t when the predetermined time T elapses, the operations shift to usual low-frequency and square-wave lighting.
However, in the technology shown in FIG. 8, there occurs fading of the discharge lamp La, in which, though the discharge lamp La is lighted in the first square-wave operation FB2, the discharge lamp La is not lighted in the subsequent period of the high-frequency operation HF1.
In Japanese Patent Laid-Open Publication No. 2004-265707, it is described that, at the time of starting a high-brightness discharge lamp, a section in which a high voltage is applied by a resonant operation and a section in which a low-frequency square wave voltage is applied are repeated alternately. In accordance with this technology, such a dielectric breakdown between the electrodes in the section in which the high voltage is applied by the resonant operation is ensured, and the shifting from the glow discharge to the arc discharge is ensured by the section in which the low-frequency square wave voltage is applied.
In the technology described in Japanese Patent Laid-open Publication No. 2004-265707, also after the discharge lamp was lighted once, such a generation period of the high frequency voltage and such a generation period of the square wave voltage are repeated alternately. In this case, since the resonant inductor L2 has a high impedance with respect to the high frequency, the resonant inductor L2 becomes a large current restriction element during the generation period of the high frequency voltage, causing the fading of the discharge lamp to be induced.
The present invention has been made in consideration for the points as described above. It is an object of the present invention to ensure secure startability of the discharge lamp in the discharge lamp lighting device that alternately generates the high frequency voltage and the square wave voltage at the time of starting the discharge lamp.